In solid structures, particularly in load-bearing structures of, for example, bridges, buildings, tunnels, railways, containment walls, dams, embankments, pipelines and underground structures of metropolitan transport lines, and so on, it is very important to monitor, in many points, significant parameters, like, for example, pressure, temperature and mechanical stresses. Such monitoring is carried out periodically or continuously, and is useful both at the initial stage and during the lifetime of the structure.
For this purpose, an approach in this field includes application of electronic monitoring devices based on electronic sensors, capable of providing good performance at low cost. Usually, such devices are applied onto the surface of the structures to be monitored, or inside recesses already foreseen in the structure and accessible from the outside.
Such devices are not however able to exhaustively detect the parameters within the structure to be monitored, which it may be useful to know in order to evaluate the quality of the structure, its safety, its ageing, its reaction to variable atmospheric conditions, and so on. Moreover, such devices can only be applied after the structure has been built, and not while it is being built. Therefore, they are unable to evaluate possible initial defects.
An approach to these requirements disclosed in U.S. Pat. No. 6,950,767 to Yamashita et al., which provides an electronic monitoring device entirely contained, i.e. “buried”, within the material (for example, reinforced concrete) from which the structure to be monitored is made. More specifically, the device buried in the structure is an entire system encapsulated in a single container, made up of different parts, assembled on a substrate, such as integrated circuits, sensors, antenna, capacitors, batteries, memories, control units, and yet more, made in different chips connected together through electrical connections made with metallic connections. This approach is of the type System-in-Package (SiP), in which the SiP is coated with a casing of mold material, such as an epoxy resin. Such a system communicates with the outside by way a radio communication sub-system contained therein, which must have an antenna sized in such a way to be able to communicate with a remote system.
The system of U.S. Pat. No. 6,950,767 to Yamashita et al. also comprises sub-systems having functions correlated with the power supply, for example, rectifiers in the case in which it receives energy from the outside, through electromagnetic waves, or else its own battery for generating the power supply internally. It should be observed that a monitoring system intended to be “embedded” initially in a building material (for example, liquid concrete, which will then solidify) and to then remain “buried” in the solid structure, is subjected to critical conditions, for example, extremely high pressures, which can even be a few hundreds of atmospheres. There are also numerous other causes of wearing, over time, due, for example, to water infiltration, capable of damaging the system.
A potential drawback to systems, such as that in U.S. Pat. No. 6,950,767 to Yamashita et al., derives from the fact that they are complex systems, even though they are enclosed in a package, and can therefore be damaged when facing the operating conditions in which they must work. In particular, the electrical interconnections between the various parts of the SiP can be vulnerable, faced with the mechanical stress that the SiP inserted in the structure must withstand.
Moreover, the “window” that must be left in the package to allow the sensor to detect the relative parameter can be a weak point for possible infiltrations of humidity. Furthermore, a crack or imperfection in the coating material can allow water to penetrate inside the SiP and cause short-circuits. In addition to water, other substances, such as potentially corrosive acids, can also infiltrate.
In general, although designed for the mentioned use, the reliability of systems like that of U.S. Pat. No. 6,950,767 to Yamashita et al. has a limitation due to the complexity of the structure of such systems, although miniaturized. It should also be observed that this approach may provide the use of numerous SiPs in different points of the structure to be monitored, but in positions that are not known. The positions must be estimated based on techniques like trilateration. This can cause various drawbacks, including: if the estimation of the position of the sensors is incorrect, the data obtained by the external remote data collection and processing system can be inaccurate or difficult to interpret; and if the structure is made from reinforced concrete, and comprises metallic structures (for example, steel rods), it is impossible to control and avoid electromagnetic shield effects, and this can jeopardize the communication with the outside and/or the detection of the position of the different SiPs.